1. Field of the Invention
The present invention relates to a magnetoresistance device, and more particularly relates to a magnetoresistance device having a structure where at least one of free and fixed magnetic layers is composed of a plurality of ferromagnetic layers separated by one or more non-magnetic layers.
2. Description of the Related Art
Magnetoresistance devices, such as a memory cells of MRAM (Magnetic Random Access Memory) and magnetic heads of recording devices is often composed of a structure provided with a plurality of ferromagnetic layers whose neighbors are separated by a non-magnetic layer; such structure is referred to as the layered ferromagnetic structure, hereinafter. The layered ferromagnetic structure is designed so as to attain desirable functions by using exchange coupling between the adjacent ferromagnetic layers.
One example of the applications of the layered ferromagnetic structure is the MRAM including memory cells in which free magnetic layers are composed of SAFs (Synthetic Anti-Ferromagnet). The SAF denotes a layered ferromagnetic structure in which adjacent ferromagnetic layers are antiferromagnetically coupled. FIG. 1A is a sectional view showing one example of the memory cell structure of the MRAM in which the free magnetic layers are composed of an SAF, and FIG. 1B is the top view thereof.
As shown in FIG. 1A, an MTJ element (magnetic tunnel junction element) 101 is provided at an intersection of a word line 102 and a bit line 103. The MTJ element 101 is composed of an antiferromagnetic layer 104, a fixed magnetic layer 105, a tunnel barrier layer 106 and a free magnetic layer 107.
The free magnetic layer 107 is composed of an SAF. Specifically, the free magnetic layer 107 is composed of ferromagnetic layers 108, 110, and a non-magnetic layer 109 placed therebetween. The ferromagnetic layers 108, 110 are antiferromagnetically coupled by the exchange coupling through the non-magnetic layer 109. As shown in FIG. 1B, the MTJ element 101 is long in the direction in which the word line 102 is extended. The easy axes of the ferromagnetic layers 108, 110 are directed in the direction in which the word line 102 is extended.
Referring back to FIG. 1A, the ferromagnetic layers 108 and 110 have different magnetizations M1 and M2, respectively. This implies that a net magnetization MR of the entire free magnetic layer 107 is not zero even when an external magnetic field is not applied. The net magnetization MR of the free magnetic layer 107 can be reversed by applying the external magnetic field greater than a switching magnetic field Hc. The MTJ element 101 stores one-bit data as the orientation of the net magnetization MR of the free magnetic layer 107.
FIG. 1C is a magnetization curve of the free magnetic layer 107 thus structured. In the range where an external magnetic field H is relatively small, the antiferromagnetic coupling is maintained between the ferromagnetic layers 108 and 110. The free magnetic layer 107 exhibits the behavior similar to that of a single ferromagnetic layer. Specifically, the free magnetic layer 107 exhibits hysteresis characteristics in the magnetization curve. The net magnetization MR of the free magnetic layer 107 can be reversed by applying the external magnetic field greater than the switching magnetic field Hc.
The magnitude of the switching magnetic field Hc depends on the direction of the external magnetic field, namely, the magnitudes of the components of the external magnetic field in the directions of the easy and hard axes. In detail, as shown in FIG. 1D, the switching magnetic field Hc exhibits an asteroid curve in a coordinate system where the magnetic field in the easy axis direction is represented on the horizontal axis, and the magnetic field in the hard axis direction is represented on the vertical axis.
Referring back to FIG. 1C, the net magnetization MR of the free magnetic layer 107 is not increased by the application of the external magnetic field that slightly exceeds the switching magnetic field Hc. This is because the magnetizations of the ferromagnetic layers 108 and 110 are kept antiparallel by the antiferromagnetic coupling between the ferromagnetic layers 108 and 110, and the increase in the external magnetic field does not contribute to the increase in the net magnetization MR of the free magnetic layer 107.
When an external magnetic field HE is further increased to exceed a certain magnetic field, the net magnetization MR of the free magnetic layer 107 begins to be increased. This is because the directions of the magnetizations of the ferromagnetic layers 108, 110 are redirected against the antiferromagnetic coupling, and the magnetizations of the respective ferromagnetic layers are placed out of the antiparallel state. Hereafter, in this specification, the magnetic field at which the magnetizations of the ferromagnetic layers within the SAF are placed out of the antiparallel state is referred to as a threshold magnetic field H1. When the external magnetic field exceeding the threshold magnetic field H1 is applied, the magnetizations of the ferromagnetic layers 108 and 110 are neither parallel nor anti-parallel. The angles of the magnetizations of the ferromagnetic layers 108, 110 depend on the magnitude of the external magnetic field.
When the external magnetic field HE is further increased, the magnetizations of the ferromagnetic layers 108 and 110 are directed in parallel, and the net magnetization MR of the free magnetic layer 107 is saturated. After the magnetizations of the ferromagnetic layers become parallel, there is no increase in the net magnetization MR caused by the changes in the directions of the magnetizations of the respective ferromagnetic layers, and the magnitude of the net magnetization MR of the free magnetic layer is no longer increased.
Data is written onto the free magnetic layer 107 within the SAF by sending write currents through both of the word line 102 and the bit line 103, and thereby applying a magnetic field to the free magnetic layer 107, similarly to typical MRAMs. Since the write currents are sent through both of the word line 102 and the bit line 103, a synthetic magnetic field is generated in a direction oblique to the easy axis, ideally, in the direction of the angle of 45° to the easy axis. The directions of the write currents, that is, the direction of the synthetic magnetic field is determined in accordance with the data to be written. Depending on the generated synthetic magnetic field, the magnetizations of the ferromagnetic layers 108 and 110 within the free magnetic layer 107 are flipped to desired directions to write the desired data onto the free magnetic layer 107.
The magnitudes of the write currents through the word line 102 and the bit line 103 are selected so that the synthetic magnetic field exceeds the switching magnetic field Hc. Specifically, the direction and magnitude of the synthetic magnetic field are selected such that the synthetic magnetic field corresponds to a point outside the asteroid curve in the coordinate system shown in FIG. 1D. Under such meaning, the above-described writing operation is referred to as the asteroid writing, hereinafter. When the asteroid writing is used, the antiparallel coupling between the adjacent two ferromagnetic layers is required to be maintained within the SAF of the free magnetic layer. This requires the sufficiently large exchange coupling, namely, the sufficiently large threshold magnetic field H1.
Spin-current injection may be used for bit writing instead of the asteroid writing, which involves magnetization reversal of the free magnetic layer 107 by injecting a spin-polarized current into the free magnetic layer 107 and thereby transferring a spin torque of the spin-polarized electrons. In the structure shown in FIG. 1A, the spin-polarized current through the tunnel barrier layer 106 in the vertical direction exerts a spin torque between the free magnetic layer 107 and the fixed magnetic layer 105. The directions of the magnetizations are controllable by the direction of the spin-polarized current. The switching current of spin-current injection also depends on the switching magnetic field Hc of the SAF. The use of spin-current injection for the MRAM is advantageous for reducing the write current and avoiding writing errors. In particular, a single magnetic domain is easily established in the respective ferromagnetic layers within the SAF, and the net magnetization MR of the SAF can be easily reduced. This is advantageous in the spin-current injection.
Another example of the application of the layered ferromagnetic structure is a toggle writing MRAM disclosed in U.S. Pat. No. 6,545,906. In this MRAM, differently from the MRAM adopting the asteroid writing, a free magnetic layer is composed of an SAF exhibiting a net magnetization of substantially zero when no external magnetic field is applied.
FIG. 2A is a plan view showing the structure of the MRAM disclosed in U.S. Pat. No. 6,545,906. The MRAM has a free magnetic layer 201, a word line 202 extended at an angle of 45° to the easy axis of the free magnetic layer 201, and a bit line 203 orthogonal to the word line 202. The free magnetic layer 201 is composed of an SAF having a free magnetic layer composed of two ferromagnetic layers having the same magnetization.
FIG. 2B is a graph showing the magnetization curve of the free magnetic layer 201. The net magnetization MR of the free magnetic layer 201 is substantially 0 when the applied external magnetic field is small. This is because the magnetizations of the ferromagnetic layers are kept antiparallel by the antiferromagnetic coupling between the ferromagnetic layers.
When the magnitude of the external magnetic field is further increased to a certain magnitude, the external magnetic field suddenly breaks the antiferromagnetic coupling between the two ferromagnetic layers, and then, the magnetizations of the two ferromagnetic layers are rearranged at a certain angle so that the direction of the resultant magnetization vector of the two ferromagnetic layers is in coincidence with the direction of the external magnetic field. Hereinafter, such magnetic field is referred to as the spin flop field Hflop. When the magnitude of the external magnetic field is further increased in the range between those of the spin flop field Hflop and the saturation magnetic field Hs, the increase in the applied external magnetic field increases the net magnetization of the free magnetic layer. This is because the directions of the magnetizations of the ferromagnetic layers are redirected to be nearly placed in the parallel state. When the applied magnetic field is further increased to then exceed the saturation magnetic field Hs, the magnetizations of the ferromagnetic layers become completely parallel, and the net magnetization of the free ferromagnetic layer is saturated.
FIG. 3 is a diagram showing the writing operation of the MRAM disclosed in the patent document 1. It should be understood that symbols M1, M2 denotes the magnetizations of the respective ferromagnetic layers within the free magnetic layer 201.
The data writing of this MRAM is achieved by rotating the in-plane direction of the magnetic field applied to the free magnetic layer and consequently rotating the magnetizations of the ferromagnetic layers within the free magnetic layer 201 to desired directions. Specifically, at first, a write current is sent through the word line 202 so that a magnetic field HWL is generated in the direction vertical to the word line 202 at a time t1. Another write current is then sent through the bit line 203 at a time t2 while the write current through the word line 202 is maintained. Consequently, a magnetic field HWL+HBL is generated in the direction oblique to both of the word line 202 and the bit line 203, typically, in the direction at an angle 45° to the word line 202 and the bit line 203. In succession, the write current to the word line is terminated at a time t3 with the write current maintained through the bit line 203. Consequently, the magnetic field HBL is generated in the direction orthogonal to the bit line 203, that is, in the direction parallel to the word line 202. The thus described process achieves rotation of the magnetic field applied to the free magnetic layer 201, and resulting in that the magnetizations of the ferromagnetic layers within the free magnetic layer 201 are rotated by 180 degrees. The data writing in this procedure may be referred to as the toggle writing, hereinafter.
In the MRAM adopting the toggle writing, the magnetic field applied to the free magnetic layer is required to be greater than the spin flop field Hflop and smaller than the saturation magnetic field Hs, when the write currents are sent to the word line 202 and the bit line 203. If not so, the magnetizations of the ferromagnetic layers within the free magnetic layer 201 are not directed to desired directions.
The MRAM adopting the toggle writing has various advantages. One advantage is that the toggle writing achieves superior selectivity. In principle, the toggle writing does not cause the rotation of the magnetizations of the ferromagnetic layers within the SAF when a write current is sent through only one of the word line 202 and the bit line 203. In other words, the magnetizations of half-selected memory cells are not undesirably reversed. This is important from the viewpoint of the operation reliability of the MRAM.
Another advantage of the toggle writing is that the tolerance for thermal activation is improved with the reduced net magnetization of the free magnetic layer. In order to improve the tolerance for the thermal activation, the volume of the free magnetic layer is required to be increased. However, in the MRAM that uses a single-layered ferromagnetic film as the free magnetic layer, the increase in the volume of the free magnetic layer undesirably increases the magnetization and thickness product (namely, the product of the magnetic film thickness and the saturation magnetization) of the free magnetic layer. as the free magnetic layer. The increase in the magnetization and thickness product of the free magnetic layer increases the magnetic field generated by the magnetization, and thereby undesirably causes the magnetic interference between adjacent memory cells. Moreover, the increase in the magnetization and thickness product of the free magnetic layer makes it hard to reverse the magnetization of the free magnetic layer. These phenomena are not preferable for the operation of the MRAM. On the other hand, the MRAM based on the toggle writing, which incorporates the SAF as the free magnetic layer, allows the volume of the free magnetic layer to be increased with a reduced net magnetization of the free magnetic layer. For example, increasing the number and/or film thickness of the ferromagnetic layers within the SAF allows the increase in the volume of the free magnetic layer. However, the net magnetization of the SAF can be ideally kept zero by using a properly designed SAF structure.
Still another example of the applications of the layered ferromagnetic structure is the fixed magnetic layer composed of two ferromagnetic layers coupled antiferromagnetically (for example, refer to Japanese Laid-Open Patent Applications Nos. P2004-87870A and P2004-253807). An advantage of such-designed fixed magnetic layer is that undesired reverse of the magnetizations is not easily caused by an external magnetic field due to the reduced net magnetization; the net magnetization of the fixed magnetic layer incorporating an SAF is ideally zero. In order to make the net magnetization of the fixed magnetic layer closer to 0, the two ferromagnetic layers are coupled in an antiferromagnetic manner, and designed to have the same magnetizations. The fact that the two ferromagnetic layers are coupled in the antiferromagnetic manner is important to provide the net magnetization of zero.
One requirement on the layered ferromagnetic structure (typically, the SAF) is that the sufficiently large exchange coupling acts between adjacent ferromagnetic layers. For example, an MRAM which uses an SAF as the free magnetic layer and performs the asteroid writing requires sufficiently large antiferromagnetic exchange coupling between the ferromagnetic layers. If not so, the free magnetic layer does not function as the SAF. Similarly, an MRAM which uses an SAF as the fixed magnetic layer requires sufficiently large antiferromagnetic exchange coupling between the ferromagnetic layers of the SAF.
Achieving sufficiently great exchange coupling may be a problem, especially in the case where a layered ferromagnetic structure is formed on a tunnel barrier layer. The tunnel barrier layer is often formed of an amorphous layer or a poorly-oriented layer, such as AlOx. As a result, a ferromagnetic layer formed on the tunnel barrier layer often exhibits poor orientation. The poorly-oriented ferromagnetic layer causes a non-magnetic layer formed thereon to be also poorly oriented. This weakens the exchange coupling between the ferromagnetic layers, and prevents desired properties from being achieved in the layered ferromagnetic structure. Such situation is especially severe when the ferromagnetic layers within the SAF are formed of NiFe. It is hard to obtain sufficiently large exchange coupling between NiFe ferromagnetic layers within SAF.
The same applies to a case where the crystalline structure of the tunnel barrier layer is not compatible for growing SAF films thereon, even if the tunnel barrier layer is formed of a highly-oriented insulating film, such as a MgO film having the NaCl crystal structure. In such situations, large exchange coupling is not obtained, which causes the same problem. In many cases, crystal structures of SAF films are not well matched with that of the underlying crystalline tunnel barrier film.
Although there is a need for a technique that achieves improved crystal growth of SAF films regardless of the crystal structure the underlying tunnel barrier, no approach has been currently proposed.
Incorporating a layer structure composed of CoFe and NiFe films within each ferromagnetic layer of the SAF may achieve enhanced exchange coupling; however, the use of CoFe films results in the increase in the saturated magnetization and crystal magnetic anisotropy of the SAF. This is not preferable for the operation of the MRAM. For the MRAM adopting the asteroid writing, the increases in the saturated magnetization and crystal magnetic anisotropy undesirably increase the switching magnetic field Hc and thereby increase the write current.
Another requirement imposed on the layered ferromagnetic structure is that the strength of the exchange coupling between the ferromagnetic layers can be easily controlled. In the toggle writing, for example, it is important that an anisotropic magnetic field Hk of each ferromagnetic layer and an exchange coupling energy J between the adjacent ferromagnetic are appropriately adjusted. This is because the spin flop field Hflop and the saturation magnetic field Hs, which determine the operational margin of the toggle writing, depend on the exchange coupling energy J. Specifically, the spin flop field Hflop and saturation magnetic field Hs of the SAF composed of two ferromagnetic layers are represented by the following equations:Hs=2J/Ms·(1/t1+1/t2)−2K/M,  (1)Hflop=2/Ms·[K(2J/t−K)]0.5,  (2)where J is the exchange coupling energy acting through the non-magnetic layer within the SAF, Ms is the saturated magnetization of the SAF, K is the anisotropic energy, and t1 and t2 are the film thicknesses of the respective ferromagnetic layers within the SAF. It should be noted that the anisotropic energy K is increased with the increase in the anisotropic magnetic field Hk, and that the saturation magnetic field Hs given by the equation (1) can be approximated by only the first term, when t1 and t2 are not equal. Moreover, the equation (2) can be defined only when t1 and t2 are equal. That is, if the equation (2) can be defined, it holds that t=t1=t2.
The equation (2) can be rewrite as shown below:Hflop=(Hs×Hk)0.5  (2)′As can be understood from the equations (1), (2), the toggle writing requires a sufficiently increased exchange coupling energy J in order that the ferromagnetic layer functions as the SAF. However, the excessive increase in the exchange coupling energy J undesirably leads to the increase in the spin flop field Hflop. Hence, the exchange coupling energy J is required to be controlled to a proper value. In addition, it would be preferable the anisotropic magnetic field Hk, namely, the anisotropic energy K can be controlled independently of the exchange coupling energy J, because it allows the spin flop field Hflop to be controlled independently of the saturation magnetic field Hs, as can be understood from the equation (2)′.
As is widely known to those skilled in the art, the exchange coupling energy that acts between the ferromagnetic layers is somewhat controllable on the basis of the thickness of the non-magnetic layer formed therebetween, as shown in FIG. 4. However, in order to stabilize the magnitude of the exchange coupling energy, the thickness of the non-magnetic layer should be adjusted so that the exchange coupling energy exhibits an extreme value. This implies that the magnitude of the exchange coupling energy is not freely controllable by the thickness of the non-magnetic layer. It is advantageous to provide a technique for controlling the exchange coupling energy through parameters other than the thickness of the non-magnetic layer for attaining highly-reliable MRAMs based on the toggle writing.
U.S. Pat. No. 6,714,446 discloses an improved SAF structure in which two ferromagnetic layers within an SAF are each composed of two ferromagnetic films antiferromagnetically coupled. The disclosed SAF structure, however, does not address enhancing or controlling the exchange coupling energy.